Over at Deliberate Parenting, there is a nice little post called It is probably not an open-ended art experience if..., and it breaks an art experience down to the core components that make it open ended. Giving very specific directions? Turning out a product that looks just like another one? Its probably not open ended.
A new-to-me site, Art Junction, shares what they call "art sparkers". They are similar to what I would think of as prompts. If you're still not quite sure about what open-ended art experiences are, this is a resource that shares some ideas to suggest and promote as you're leading children with an art experience - the beauty is in the process, not the product!
Colorful Circles is an example of a collaborative, open-ended art experience that had a "sparker" at the beginning - I joined the children at this station in the classroom to show how I traced the circles, but after that, it was up to them. There were plenty of lines and other drawings on the paper, also, but the process of working on tracing and connecting circles, and then revisiting it the next day, was wonderful.